The present invention relates to metal founding, and more particularly to a method of making a sand-based mold which improves the quality of castings by reducing veining defects, and to the metal casting prepared by said method.
Iron oxides have been used for years in foundry applications to improve core properties and the quality of castings. Iron oxides have proven to be advantageous as an additive to foundry molding aggregates containing silica sand to improve the quality of castings by reducing the formation of thermal expansion defects, such as veining, scabs, buckles, and rat tails as well as gas defects, such as pinholes and metal penetration. There are several iron oxides which are currently used in foundries today. These include red iron oxide, also known as hematite (Fe.sub.2 O.sub.3), black iron oxide, also known as magnetite (Fe.sub.3 O.sub.4) and yellow ochre. Another iron oxide which is presently being used is Sierra Leone concentrate which is a hematite ore black in color. Red iron oxide and black iron oxide are the most popular iron oxides in use.
The currently accepted method of employing the above iron oxides is to add approximately 1-3% by weight to the sand mold aggregates during mixing. The exact mechanism by which iron oxides affect surface finish is not totally understood. However, it is generally believed that the iron oxides increase the hot plasticity of the sand mixture by the formation of a glassy layer between the sand grains which deforms and "gives", without fracturing at metallurgical temperatures, to prevent fissures from opening up in the sand, which in turn reduces veining.
Various other types of additives have also been employed in an attempt to improve core properties and the quality of sand castings. For example, other anti-veining compounds which have been utilized in sand aggregate mixtures include starch based products, dextrin, fine ground glass particles, red talc and wood flour i.e. particles of wood coated with a resin. All of these additives have met with limited success in reducing veining.
Currently, minerals containing lithia are utilized in the glass, glaze, and enamel industries as a fluxing agent. Also, in Nakayama et al, U.S. Pat. No. 5,057,155 a lithium mineral is added to a mold-forming composition to function as an expansive agent during heating and firing of ceramic molds used in the investment casting industry. According to Nakayama et al, the mold-forming composition irreversibly expands during firing of the mold in proportion to the amount of lithium mineral present to provide dimensional accuracy for castings by compensating for solidification shrinkage which occurs during cooling of poured metals such as titanium and the like used, for example, in dental castings.
It should be noted that additives containing lithia have not been added to sand-based foundry molding and core aggregates. Nakayama et al fails to provide any motivation to one skilled in the art to use a lithia-containing compound such as .alpha.-spodumene as an anti-veining agent in sand-based foundry molding and core mixtures. First, one would not expect .alpha.-spodumene to work in silica sand-based aggregates because: (a) .alpha.-spodumene expands upon heating and curing of the mold slurry as taught in Nakayama et al; and (b) in silica sand-based molds expansion of the mold is undesirable since an expanded sand mold would create cracks. Thus, one would expect veining to actually be enhanced rather than reduced if an agent that expands upon heating was used in a silica sand mold. Second, one would not expect .alpha.-spodumene to be effective in sand castings due to its relatively low melting temperature. In other words, since a-spodumene melts at a temperature less than the pouring temperature of the metal, no one would want to add such a material to silica sand castings since it would be expected that such a material would melt and thus change its form and shape during pouring. Third, the composition of Nakayama et al is a slurry rather than a discrete shape. Clearly, as a slurry, it could not be employed to process a mold for any sand casting operation. These slurries, once fired and turned into a ceramic mold, the mold composition has been transformed and no longer contains .alpha.-spodumene but instead contains .beta.-spodumene which is totally different in crystal structure. The use of lithium-containing minerals, such as a-spodumene, for a lithia source as an additive to sand-based foundry aggregates thus is a unique application.